Method for recovering metallic elements, especially metallic chromium, from slag containing metal oxides in an electric-arc furnace

ABSTRACT

The invention relates to a method for recovering metallic elements, especially metallic chromium, from slag containing oxides, especially chromium oxides, in an electric-arc furnace. According to said method, the slag is not reduced in a separate step after melting, but the following steps are carried out: a charge is introduced into tile electric-arc furnace, and melted, forming a molten metal and slag; said molten metal is tapped off, leaving the unreduced slag in the furnace vessel; a further charge of scrap metal including reducing agents for tile slag is added; the slag is reduced during the melting of said charge; and the slag and the molten metal are then lapped off. The inventive method can also be used in ladle metallurgy or converter metallurgy.

The invention concerns a method for recovering metallic elements, especially metallic chromium, from slags that contain oxides, especially chromium oxides, in an electric arc furnace, a converter, and in ladle metallurgy.

During the melting down of metal scrap in arc furnaces, oxidation processes occur in which the metallic elements of the melt react to oxides. In the case of high-grade steels, such as chromium steels and chromium-nickel steels, it is mainly the expensive chromium which is oxidized. Parallel to this process, the oxides are also reduced again. In the case of high-chromium steels, especially chromium oxide reduction with carbon constitutes a fundamental mechanism. In this regard, the primary oxidation of the chromium is compensated by a parallel reduction with the carbon dissolved in the melt. This reaction takes place at the bubble surface and at the metal/slag phase boundary. The product of the reactions, metallic chromium, is returned to the melt. The carbon monoxide that forms is carried off into the furnace atmosphere after diffusion in bubbles at the surface.

The following reactions occur: {O₂}=2 [O] dissociation  (1) [C]+[O]={CO} direct decarbonization  (2) 2[Cr]+3[O]=(Cr₂O₃) chromium oxidation  (3)

These reactions occur under the following thermodynamic equilibrium: $\begin{matrix} {{K\quad(T)} = \frac{a_{Cr}^{2}p_{CO}^{3}}{a_{c}^{3}a_{{Cr}_{2}O_{3}}}} & (4) \end{matrix}$ where $\begin{matrix} {{\log\quad K\quad(T)} = {{- \frac{40536}{T}} + 25.63 + p}} & (5) \end{matrix}$ in which p is a parameter.

Due to the incompleteness of the chromium reduction with the carbon, portions of the chromium oxide in the form of various spinels enter the slag. Depending on the type of process management, the chromium oxide content of the slag varies and is more or less above 5%. The economy of the process is based on reducing the chromium oxides and recovering the metallic chromium.

This is usually accomplished by carrying out a separate reduction of the slag with high-affinity silicon in the form of FeSi at the end of the meltdown process.

WO 00/79014 A1 describes a solution to the problem of making the process of recovering metallic chromium from slags that contain chromium oxide more economical in converter processes or vacuum processes. It is proposed that the conventional treatment step of slag reduction be omitted. Slag that is present, for example, after an oxygen-blowing operation in a converter or in a ladle is removed unreduced from the metal melt and charged to an upstream arc furnace. With the addition of carbon and with silicon from the scrap charged to the arc furnace, possibly with residual dusts, the chromium oxide in the slag is then directly reduced to metallic chromium during the meltdown process.

WO 02/33130 A1 describes a process in which metallic chromium is recovered in a metallurgical vessel in which both a melting process and a blowing process are carried out, especially by the conarc process. In this process, slag that forms after the first blowing process remains unreduced in the vessel. The slag, together with the next charge, is heated in the same vessel, reduced, and then tapped. The next blowing process then takes place, and the slag which now forms is again left in the vessel.

Proceeding from this prior art, the objective of the invention is to make available an economical process for the reduction of oxides, especially chromium oxides, in the slag that forms in electric arc furnaces after the meltdown process.

This objective is achieved by the methods with the features of claims 1 and 6. Advantageous refinements of the method are specified in the dependent claims.

The basic idea of the invention is to dispense with the—previously customary—slag reduction after the melting operation to obtain the first melt. The treatment step of slag reduction is not carried out until after the second melting of a second charge or additional charges.

In detail, the following process steps are carried out: A scrap charge is introduced into the arc furnace and melted down to produce a metal melt and slag. The conventional slag reduction step is then omitted. The melt is tapped, and the slag, which has a high chromium oxide content and is saturated with manganese-iron oxide, remains unreduced in the furnace vessel.

The next charge is then charged onto this slag. This charge consists of scrap and reducing agents for the slag, especially carbon and/or silicon. It is advisable to add the carbon and/or silicon in the form of carbon-rich and silicon-rich alloying materials. Carbon and silicon are contained, for example, in charge chromium, ferrochromium, or ferronickel. They are sufficient for most of the reduction. In certain cases, aluminum or additional carbon and/or silicon additives, such as FeSi, may be added. Reducing agents can also be contained in residual dusts that are charged to the furnace.

During the melting process produced by electric power, the slag is directly reduced with the aid of these reducing agents. Direct reduction of the chromium oxide with carbon and/or silicon takes place in the arc furnace under atmospheric pressure. The metal oxidation that occurs during the melting process is (over)compensated by reduction reactions, and the metallic chromium is recovered. The slag is then removed. An analogous process takes place in ladle metallurgy, in which the metal is already present as a melt.

The process in the electric arc furnace is considerably improved by carrying out the method with the use of residual metal bottoms. The reduction quality of the slag is considerably improved by metal bottoms. In this case, the first melt is not completely tapped but rather is only partially tapped to leave metal bottoms in the melting vessel. This has the advantage that the slag floating on these metal bottoms remains in the furnace and thus cannot become stuck to or caked on the bottom of the furnace. In this advantageous modification of the method of the invention, a melting campaign comprises a buildup melt phase, a separation melt phase, and a repeating phase of meltdown and reduction that occurs between the buildup and separation phases. In contrast to the previously known method in the conarc process, which comprises melting down and blowing with oxygen to produce steel, the present method pertains to the recovery of chromium during meltdown in an arc furnace, and in this method, the process is comparatively simpler and shorter. The chromium concentrations are also not comparable; the chromium concentrations of the slag during meltdown are lower, e.g., on the order of 14-15%.

The proposed method is used especially with high-grade steels that contain chromium or chromium-nickel. Depending on the overall technology, i.e., a high-grade steel technology without a separate reduction treatment of the slag in an electric arc furnace, this process can be optimized with respect to costs and power consumption. It is unimportant for the method whether the slag is in a solid, liquid, or intermediate state. This has an effect only on the meltdown time and the power consumption in the arc furnace.

The thorough mixing necessary for the reduction during the melting process is preferably produced by inert gases, which are introduced through bottom nozzles and/or side nozzles in the arc furnace and/or through a top lance. The reaction surface between the slag and the metal is increased in this way.

All together, the proposed method has the following advantages:

shortening of the total treatment time of the melt by up to 15 minutes, depending on the type of technology by elimination of the conventional separate reduction step;

reduction of FeSi consumption;

reduction of the consumption of fluxes;

high metal melt output;

increased service life of the refractory material of the furnace;

increased service life of the porous plugs/nozzles;

improved energy balance of the electric arc furnace; and

improved degree of purity of the metal

The individual steps of the method are described in detail below with reference to the sole figure in the form of a process flowchart, which shows the working of the metal in the EAF or electric arc furnace process.

In a first step, a buildup melt without bottom product is produced by melting a scrap charge to form a metal melt and a slag. The melt is tapped, but only partially, so that metal bottoms with slag floating on top remains in the furnace vessel. The slag is unreduced.

In the melting and reduction phase that follows, scrap is charged onto these metal bottoms with slag. In addition, reducing agents for the slag, such as carbon and silicon, are added. During the meltdown process, the slag is reduced. The following reduction reactions with carbon and silicon take place: (CR₂O₃)+3[C]=2[Cr]+3{CO} 2 (Cr₂O₃)+3[Si]=4(Cr]+3(SiO₂)

These reactions lower the chromium oxide concentration of the slag to values below the initial value.

The now reduced slag is tapped. In the further course of the process, the melt is tapped, but again only partially, to leave metal bottoms at the bottom of the furnace vessel. Further material is then charged onto these metal bottoms. The slag that forms during the meltdown process is directly reduced again. These steps are repeated for each melt of a campaign. When the campaign has been completed, instead of a partial tapping of the melt, the melt is totally tapped, together with the metal bottoms.

The method can be carried out in an electric arc furnace or other types of melting furnaces as well as in ladle metallurgy or converter metallurgy or other metallurgical vessels. 

1. Method for recovering metallic elements, especially metallic chromium, from slags that contain oxides, especially chromium oxides, in an electric arc furnace, with the following steps: (a) introduction of a charge into the arc furnace; (b) meltdown of the charge to produce a metal melt and slag; (c) tapping of the melt while leaving the unreduced slag in the furnace vessel; (d) introduction of another charge that contains scrap and reducing agents for the slag, such as carbon and/or silicon and/or aluminum; (e) meltdown and direct reduction of the slag, especially direct reduction of chromium oxide, by the reducing agents in the charge during the melting process; (g) tapping of the melt and the reduced slag.
 2. Method in accordance with claim 1, comprising the melting down of a buildup melt at the beginning of a melting campaign; a partial tapping of the melt in step (c) while leaving metal bottoms in the furnace vessel; the charging of another charge onto the remaining metal bottoms and the slag; a partial tapping of the metal melt after the meltdown of the charge and the reduction of the slag; and repetition of the steps of charging, melting, and slag reduction for each melt of the campaign or termination by total tapping of a separation melt together with the metal bottoms.
 3. Method in accordance with claim 1, wherein the reducing agents carbon and/or silicon and/or aluminum are charged in the form of alloying materials with a high carbon and silicon content.
 4. Method in accordance with claim 1, wherein residual dusts are charged as reducing agents for the slag, especially a chromium-containing slag.
 5. Method in accordance with claim 1, wherein thorough mixing is carried out by means of inert gases, which are introduced through bottom nozzles and/or side nozzles in the arc furnace and/or through a top lance.
 6. Method for recovering metallic elements, especially metallic chromium, from slags that contain oxides, especially chromium oxides, in ladle metallurgy or converter metallurgy, with the following steps: (a) introduction of a first metal melt with slag into a ladle or a converter; (b) after the treatment of the melt, especially with oxygen, tapping of the melt while leaving the unreduced slag in the ladle or converter; (c) introduction of another melt together with reducing agents for the slag, such as carbon and/or silicon and/or aluminum; (d) thorough mixing of the melt with the slag, resulting in direct reduction of the slag, especially chromium oxide, by the reducing agents; (g) tapping of the melt and the reduced slag.
 7. Method in accordance with claim 6, comprising a partial tapping of the melt in step (b) while leaving metal bottoms in the ladle; the charging of another melt onto the remaining metal bottoms and the slag; a partial tapping of the metal melt in step (g) after reduction of the slag; repetition of the steps of charging and of slag reduction for each melt of the campaign or termination by total tapping of a separation melt together with the metal bottoms. 